Back again with another question, from my ignorance. Attached are two transformer schematics. They both have rectifiers coming off the secondaries. The question is one of these center tapped or both? I'm in the process of figuring out what transformer to buy, and the nomenclature is confusing.

1 is providing isolated power supplies with its two secondary windings.
2 is essentially doing the same thing, but there is no isolation. Also, the common is not at the same potential, so this configuration is a little odd and non-traditional.

Both of these xfmrs could be used as a center tapped transformer if connected properly. If you want a +/- dual supply, you only need a total of 4 diodes with a center tap. Do not wire as per #2. #1 could work, but is a little wasteful of voltage and has extra diodes.

Tell us what you are trying to achieve. Your post 5 could be interpreted as a requirement for two DC voltage rails, one at twice the voltage of the other. If so, you need a CT secondary (or two separate identical secondaries, wired as a CT), one bridge and pairs of caps. Wire the reservoir caps like a dual +- supply, ground the -ve instead of the '0V'', then add the resistors and smoothers to the voltage rails.

Dual secondary is one way of winding a transformer.
Two independent windings that are isolated from each other.
This would be specified as 0-20, 0-20Vac, or dual 20Vac
Centre tapped is an alternative way of winding a transformer.
one long winding that has a tapping in the centre. This results in a winding that has 3 "tails". They are not isolated from each other. Since one of tails is connected to the centre/middle of the long winding the voltages in either half of the winding are identical. This is usually specified as 20-0-20Vac or as 40Vac centre tapped.

__________________
regards Andrew T.
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